"The Daniel book and the two-volume Isaiah's Prophecy books seemed to close out the era of studying the "deeper things"."
donuthole and Magnum reminded me of comments I heard when I was in bethel. The Daniel book was fairly well received, but there was a lot of disappointment that no predictions for the future were made for things like King of the North. It was more of a historical review rather than something motivating. And for the two Isaiah books, there were a lot of complaints from publishers and book study conductors. These things were so painfully slow, boring, and repetitious that I know our congregation saw new lows in attendance for CBS.
And as noted, the "deeper" WT studies tended to require "new light". I think it was the last WT I ever conducted that was about several of Jesus illustrations. The lessons required explaining what WT used to teach and then the nulite that they should believe now. One of them was the illustration about the leaven in the loaf not meaning christendom anymore, but now the "invisible growth" in the JWs. Something like that. It was all rather confusing. The writing was haphazard and the sheeples wouldn't remember a word of it unless they had the WT right in front of their faces.
In case anyone hasn't seen these, it's a pretty accurate example of the "dumbing down" of the meetings: